The invention relates to a method and a device for feeding gases or gas mixtures into a liquid, suspension or emulsion in a reactor in a specific manner.
Carbon dioxide arises as a waste product in a number of technical processes, especially during the combustion of fossil fuels (coal and coke) with excess air, during the burning of lime or during the production of synthesis gas (gasification of coal and steam reforming). The synthesis gas is washed, for instance according to the Rectisol method, for use in the synthesis of ammonia or methanol as an example; carbon dioxide can be recovered in a very pure form in the process. Carbon dioxide is compressed, liquefied or frozen and precipitated in the form of dry ice at the production site for easier transport.
Although there are a few possibilities for the use of carbon dioxide, e.g. in the food industry (dry ice cooling, carbonated beverages and carbonic maceration), in the chemical industry (urea synthesis and the Kolbe-Schmitt reaction to form salicylic acid), during welding as a protective gas or for use in fog machines, consumption is significantly less than the amount produced. Methods for the use of carbon dioxide in efficient materials management above all have become more important in the last few years because of the increasing climate change caused by the greenhouse gas carbon dioxide and the increasing global population. The production of micro-algae is currently the most promising method for materials-management or energy-related use because of the approximately 100 times greater creation rate of biomass compared to terrestrial plants. Biomass, which can be used in a variety of applications (e.g. carotenoids, lipids and/or proteins), is created via photosynthesis from carbon dioxide, water and (sun)light.
A number of photobioreactors and process concepts have been developed throughout the world up to now; the closed photobioreaction system has increasingly gained acceptance because the entry of germs, fungi, bacteria and similar contaminants in it is virtually ruled out. In the area of closed systems, flow-through tube systems, especially double tube systems, demonstrate significant advantages with regard to dead flow zones, bio-film buildup, rotting, cleaning times, energy efficiency, heat transfer, heat control and biomass growth vis-a-vis plate or bag systems. What has been common to all closed systems up to now is the addition of carbon dioxide or gases containing carbon dioxide (smoke gas) from pressure containers or pressurizing units, for instance compressors and blowers. Carbon dioxide is exclusively used in terms of material-management aspects. The reaction system is cooled by means of a separate cooling circuit in small systems (the temperature of the cooling water is controlled by thermostats, for instance) and by means of evaporation cooling, as an example, in large systems (water is sprayed onto the photobioreaction system). Bio-film buildup on the walls of the reactor is a problem with regard to photobioreactors that has not been solved up to now; fairly long standstill periods of the system result from that because of shorter cleaning intervals, and there are lower biomass growth rates up to a possible complete loss of the biomass because of rotting processes.
Furthermore, a biomass dry weight content of approx. 5 g/l is achieved in customary photobioreactors at present. Flocculation and settling take place on the reactor walls with higher concentrations, which leads to a situation in which light can no longer get into the interior of the reactor.
This effect is counteracted by the use of high flow velocities, which requires the use of greater pumping power. In addition, abrasive cleaning particles are added to the system. They substantially increase the technical effort for suspension separation and simultaneously minimize the photoactive reaction volume.
This procedure for achieving efficient production methods is absolutely necessary in the case of micro-algae production. Otherwise, cellular respiration (dark reaction) increasingly takes place instead of the desired photosynthesis (light reaction), which can lead to the death of the culture in the end. A limitation of the maximum biomass dry weight concentration to approx. 5 g/l also follows from the amount of energy required to thoroughly mix the system to supply sufficient light to all of the micro-algae. Furthermore, a small concentration of micro-algae in the suspension requires a substantial amount of energy for the separation and drying of the micro-algae. An efficient procedure for the production of micro-algae therefore brings about the necessity of higher concentrations of the biomass dry weight before the treatment processes.
A number of reactions to create products are used in the material-transformation industry that have the prerequisite of an intensive mixing of the reactants, especially when gases are used in liquids. These operating processes frequently require the use of complicated technical systems or they are associated with a great deal of energy use. Systems of that type especially have critical requirements in biotechnology, where contamination-free or low-contamination operating processes are important. As an example, photobioreactors require elaborate systems to minimize the bio-film buildup at the areas where light enters and to consequently ensure on the whole that there is production of biomass over time periods of several weeks. Various methods are known with regard to this in the prior art:
For example, U.S. Pat. No. 6,220,822 B1 describes an airlift reactor that is essentially comprised of a pipe filled with a liquid into which air can be blown. The air bubbles ascend through the pipe into a discharge port and come up against a tilted baffle in this port. A flow of the liquid in the discharge port is forced in the same direction because of the upwards movement of the gas bubbles on the surface of the baffle. In addition to baffles, conical parts are also proposed that likewise bring about a flow in the discharge port.
U.S. Pat. No. 4,649,117 describes a reactor that uses the airlift principle, for instance, for a more effective harvest of cells from fermenters/bioreactors. Optimal circulation of the liquid without the use of mechanical stirring units with a minimum of mechanical shearing force is described as the essential advantage of this application. The compressed gas that is used consists of air with a share of carbon dioxide of approximately 5%. As per the invention, the gas bubbles fed in at the base of the reactor go straight upwards. The reactor content is around 5-7 liters.
Furthermore, US 2009/0303829 A1 discloses the use of a flexible, double-walled, inflatable plastic sheeting in the form of a tube for feeding in air that is anchored in the middle of the base of the storage container. The gases that are supplied could be air, oxygen, carbon dioxide or other gases.
WO 99/25657 describes a bioreactor with good mixing for the aerobic treatment of aqueous waste with a high proportion of organic and solid components using the airlift principle. In addition to the “airlift pump”, a diffuser is also employed to distribute the gases that are used, e.g. oxygen, nitrogen and ammonia. WO 99/25657 exclusively describes the treatment of aqueous waste; the gases that are used are converted via redox reactions.
U.S. Pat. No. 7,629,167 B2 discloses flexible, disposable bioreactors in the form of containers/bags/sacks made of plastic. The flexible and exchangeable bioreactor is surrounded by a solid vessel, for instance a tank, in the process. Various fittings/connectors are described for the exchange of liquids and/or gas. Moreover, sensors can be used to monitor the bioreactor. Baffles or other parts over the gas inlet opening can be used to generate flows in the bioreactor. (Condensed/pumped) gases that are used could be air, oxygen and/or carbon dioxide.
US 2005/0098497 A1 describes a reactor that is essentially characterized in that it is filled with a liquid or suspension. If there is a suspension, a gas is supplied via an immersed pipe and distributed via diffusors, so the density of the liquid-gas mixtures is substantially lowered and the solid settles. The application possibilities of a separation of solids and liquids that are stated in US 2005/0098497 A1 result from that. The separation effect is increased by the fact that the gas can be added in a pulsating fashion. Furthermore, various modifications (installation of filters/packed bed into the reactor, baffles and various diffusor geometries) are cited.
WO 2011/048108 A2 describes a tube photobioreactor, e.g. for the production of micro-algae, with a truncated-cone-shaped core structure and one or more transparent or translucent tubes. The tube is wound onto the base frame in a helical fashion and particularly distinguishes itself by the fact that it has at least two chambers. A cultivation medium flows through at least one chamber, and a heat-exchange medium flows through at least one chamber. The tube material is made of plastic or glass, preferably silicones. The bio-film buildup and therefore the system standstill periods because of cleaning work are minimized because of that. The conveyance of the cultivation medium takes place in the tube by means of an airlift in the process, i.e. takes place by means of air or by means of an air-CO2 mixture or nitrogen as a carrier gas, which simultaneously ensures the supply of the cultivation medium with CO2. The airlift principle is based on the feed-in of finely distributed gases (airlift pump). But the supply of CO2 or gases containing CO2 can also take place, separately and pulsed, via a combined system or in the upstream area of the pump and therefore serve to set the pH value in the cultivation medium.
The regulation of the pH value by the gases that are used requires a large surface area of the phase boundaries between them and the liquid, however, which has the prerequisite of a gas that is distributed as finely as possible in accordance with the prior art.
The prior art has the drawback, in addition to the limited biomass concentration of approx. 5 g/l due to bio-film buildup, that corresponding cleaning efforts and therefore reactor standstill periods or increased effort to separate the biomass dry weight, for instance when abrasive cleaning particles are added, is necessary.
A method that ensures an increase in the concentration of the biomass dry weight over 5 g/l and a simultaneous reduction of the bio-film buildup in a simple way, and consequently a reduction in the standstill periods of bioreactors, would be extremely desirable.
The task of this invention therefore consists in describing a method that overcomes the drawbacks in the prior art and that makes an increase in the biomass concentration over 5 g/l possible in a simple way with a simultaneous reduction in the bio-film buildup.